1. Field of the Invention
The present invention relates to a magnetic recording medium, and particularly to a tape-type magnetic recording medium having an extremely small overall thickness, such as 7 xcexcm or less.
2. Prior Art
With respect to a magnetic recording medium, it is desired that the overall thickness be maintained as small as possible for the purpose of attaining a large recording capacity. In addition, many attempts have been made to obtain a magnetic layer =having a smooth surface, since a high recording density has also been demanded. For example, Japanese Patent Application Kokai No. 6-68449 discloses that a magnetic recording medium having an extremely smooth and even surface and a remarkable improvement in electromagnetic transducing property by preparing a substrate having a value of 10 nm or less as obtained by subtracting Ra (Cut off: 0.08 mm) from Ra (Cut off: 0.25 mm) which results in the suppression of surface undulations, applying a lower or undercoat nonmagnetic or magnetic layer on the substrate which offsets the surface roughness of the substrate, and forming an upper magnetic layer which is applied by Wet on Wet method on the lower layers. Japanese Patent Application Kokai No. 10-204188 discloses a heat-resistant synthetic resin film in which fine particles having a specific particle size are dispersed, wherein the film has mild undulations across the surface of the film analogous to two dimensional sine curves by regulating the centerplane average roughness (SRa). However, there are no mentioning to the application to a magnetic recording medium, and the value of SRa is outside that of the present invention, thus the desired effect cannot be expected.
On the other hand, the material for the substrate disclosed in Example of Japanese Patent Application Kokai No. 6-68449 is polyethylene terephthalate (PET) and when the thickness of the substrate is reduced, stiffness becomes unsatisfactory. In addition, when the difference in Ra of the substrate is regulated as mentioned above in order to suppress undulations and the thickness of the substrate is reduced, clogging of the gap at the rotary head is frequently observed.
Accordingly, the object of the present invention is to provide a film-type magnetic recording medium having a overall thickness of 7 xcexcm or less which prevents clogging and exhibits excellent electromagnetic transducing property.
The above-mentioned problems can be solved by the present invention which is:
(1) A tape-type magnetic recording medium which comprises a nonmagnetic substrate and at least a magnetic layer thereon and has an overall thickness of 7 xcexcm or less, having a width of the tape of 6.5 mm or more and the difference in the centerplane average roughness (SRa) of the magnetic layer is in the range defined below:
3 nmxe2x89xa6SRa (Cutoff: 800 xcexcm)xe2x88x92SRa (Cutoff: 80 xcexcm)xe2x89xa615 nm.
(2) a magnetic recording medium according to the item (1) above, wherein the nonmagnetic substrate is made of an aromatic polyamide.
According to the present invention, by regulating the difference in the centerplane average roughness (SRa) of the magnetic layer in the range defined below, a magnetic recording medium which exhibits excellent durability and prevents clogging at the head, even when the overall thickness of the tape-type medium is 7 xcexcm or less, can be obtained.
3 nmxe2x89xa6SRa (Cutoff: 800 xcexcm)xe2x88x92SRa (Cutoff: 80 xcexcm)xe2x89xa615 nm
where SRa is defined as follows. A roughness surface is obtained by three-dimensionally extending a roughness curve defined by JISB0601. (JIS means Japanese Industrical Standard) and a centerplane is obtained by three-dimensionally extending a centerline also defined by JISB0601. X axis and Y axis (rectangular coordinates) are placed on the centerplane and Z axis is placed perpendicularly to the centerplane. A certain area SM (=Lxxc3x97Ly) on the centerplane is selected. Height f(x,y) of the surface as measured from the centerplane at each point (x,y) within the area SM SRa is defined by the following formula (unit: xcexcm):   SRa  =            1              S        M              ⁢                  ∫        0        Lx            ⁢                        ∫          0          Ly                ⁢                              "LeftBracketingBar"                          f              ⁡                              (                                  x                  ,                  y                                )                                      "RightBracketingBar"                    ⁢                      xe2x80x83                    ⁢                      ⅆ            x                    ⁢                      xe2x80x83                    ⁢                      ⅆ            y                              
Especially when the overall thickness is less than 6.5 xcexcm, the stiffness becomes unsatisfactory, and therefore, the conventional magnetic tape used having a width of 6.5 mm or more exhibited a poor running property when used in a video tape recorder and it was difficult to obtain a good head touch (i.e. contact condition of a magnetic tape and a head). In contrast, according to the present invention, an excellent running property and an excellent head touch can be attained even when the width of the tape is 6.5 mm or more. In this case, SRa is measured using a measuring device for three-dimensional surface roughness, and the expression xe2x80x9cSRa (Cutoff: 800 xcexcm)xe2x80x9d means that the measurement was conducted with the cutoff of 800 xcexcm.
In the same manner, the expression xe2x80x9cSRa (Cutoff: 80 xcexcm)xe2x80x9d means that the measurement was conducted with the cutoff of 80 xcexcm. By means of this measurement, undulations of the magnetic layer as a whole can be measured accurately.
When the difference in SRa [xcex94SRa=SRa (Cutoff: 800 xcexcm)xe2x88x92SRa (Cutoff: 80 xcexcm)] is below the above-mentioned range, the undulations of the magnetic layer as a whole become small, and as a result, durability of the tape and prevention of clogging are deteriorated. On the other hand, when the difference in SRa is above the range, the undulations of the magnetic layer as a whole become too large and the electromagnetic transducing characteristics tend to be deteriorated. Accordingly, it is preferred that the lower limit be 4 nm and the upper limit be 13 nm.
As effective methods to obtain xcex94SRa in the above-mentioned range, there can be mentioned: a method in which a nonmagnetic substrate having undulations of an appropriate degree is used; and a method in which a calendering roll having undulations of an appropriate degree on its surface is used, in the case that calendering is conducted after applying a magnetic layer onto a nonmagnetic substrate. There also can be mentioned: a method in which minute vibration is applied to a nonmagnetic substrate during the application of a magnetic layer; a method in which the viscosity of the magnetic coating is adjusted; and a method in which a nonmagnetic substrate containing relatively large particles is formed for the purpose of providing undulations on the surface of the nonmagnetic substrate.
As a material for the nonmagnetic substrate, an aromatic polyamide resin is especially preferred. By using this material, the stiffness as a whole can be enhanced and excellent durability and head touch can be attained even when the overall thickness of the magnetic recording medium is below 6.5 xcexcm. As an aromatic polyamide, there can be mentioned a polyamide comprising 50 mol % or more, preferably 70 mol % or more of the repeating unit represented by the following formula: 
wherein hydrogen atom(s) of the aromatic rings is optionally substituted with substituent(s), such as a halogen group and a nitro group. From a viewpoint of obtaining excellent stiffness, a polymer in which preferably 70% or more, more preferably 80% or more, of the entire aromatic rings are bonded at para-position, is preferred. To the polyamide may be added additives, such as a lubricant such as SiO2, TiO2 and CaSO4 and an antioxidant, as long as they do not deteriorate physical properties of the polyamide. Young""s modulus of the nonmagnetic substrate obtained using the aromatic polyamide, as measured at 20xc2x0 C. and 60% RH, is 19,600 N/mm2 or more, preferably 27,440 N/mm2 or more, in terms of the sum of the value measured in longitudinal direction and that measured in transversal direction.
In order to provide a nonmagnetic substrate with undulations of appropriate degree, a production method described in Example 1 can be used. With respect to the other processes, such as dispersion process and the application process of the magnetic layer and the back-coating layer, and the drying process and the slitting process conducted afterwards, conventional methods can be used, except when a method is specifically explained.
Examples of binders used in a magnetic layer, a back-coating layer and optionally a lower or undercoat layer placed between a magnetic layer and a substrate include a thermoplastic resin, a thermosetting resin, a reaction resin and an electron-sensitive modified resin. They may be used in combination and the combination is suitably selected depending on the properties of the medium and the conditions of the process.
Examples of thermoplastic resins include a combination of a vinyl chloride copolymer and polyurethane resin, a (metha)acrylic resin, a polyester resin and a nitrocellulose resin. Among these resins, a resin having hydroxyl group(s) at the terminal or the side chain thereof is preferred, since such a resin functions as a reaction resin which facilitates a cross-linking utilizing polyisocyanate and an electron cross-linking. Further, a resin may have acidic polar group(s), basic polar group(s) or the like, such as xe2x80x94COOH, xe2x80x94SO3H, xe2x80x94OSO3M, xe2x80x94OPO3M, xe2x80x94PO3M, xe2x80x94N+R3Clxe2x88x92, xe2x80x94NR2 or the like (wherein M represents H or an alkali metal, such as Na and K; and R represents H or a hydrocarbon group) at the terminal or the side chain thereof. A resin containing such group(s) is suitable for improving dispersibility. These resins may be used alone or in combination.
Among these, a combination of a vinyl chloride copolymer and a polyurethane resin is preferred. The polyurethane resin used with the vinyl chloride has advantages in abrasion resistance and adhesion to the substrate. In addition to the above-mentioned resins, conventional resins may be used.
It is possible to use a modified resin obtained by process the above-mentioned resin by a conventional method thereby introducing (metha)acrylic double bonding and modifying electron-sensitivity.
Examples of magnetic particles to be used in the present invention include: powder of magnetic metal such as iron powder containing at least one member selected from cobalt, nickel and rare earth element (including Y); and cobalt and an alloy of cobalt and nickel; powder of magnetic oxide (such as cobalt-containing iron oxide magnetic powder and barium ferrite powder); and other conventional magnetic powders. It is preferred that the particle of magnetic powder be a needle-like particle having a major-axis length of approximately 0.05-0.3 xcexcm or a hexagonal particle having a plate diameter of approximately 0.03-0.2 xcexcm.